Stimulated Brillouin scattering in single-frequency fiber amplifiers with delivery fibers

For an all-fiber-based single-frequency amplifier, a passive delivery fiber carrying the amplified signal out of the pump combiner may induce unwanted stimulated Brillouin scattering (SBS) and affect amplifier performance. To evaluate the impact, a rate-equation based model has been adopted to study SBS characteristics in the fiber amplifier with a delivery fiber. The model allows independent inputs of many critical parameters, such as the delivery fiber length, temperature distribution, core size, Brillouin gain coefficient, as well as the gain fiber length. The SBS thresholds of the amplifiers under various conditions are computed. The results indicate that the delivery fiber lengths, core diameter, and Brillouin gain coefficient are the most influential parameters which make great impacts on amplifier performance. In addition, the gain fiber length also plays a role on SBS thresholds, but its impact is modest and normally less than a few percent. For an amplifier with a 25 microm gain fiber, to achieve more than 400 W output, the delivery fiber should have a Brillouin gain coefficient <1 x 10(-11) m/W with a length of less than 2 m and a core diameter of greater than 35 microm.